A semiconductor device includes a substrate having a logic device region including logic devices thereon, and an input/output (I/O) device region including I/O devices thereon adjacent the logic device region. A first fin field-effect transistor (FinFET) on the logic device region includes a first semiconductor fin protruding from the substrate, and a triple-gate structure having a first gate dielectric layer and a first gate electrode thereon. A second FinFET on the I/O device region includes a second semiconductor fin protruding from the substrate, and a double-gate structure having a second gate dielectric layer and a second gate electrode thereon. The first and second gate dielectric layers have different thicknesses. Related devices and fabrication methods are also discussed.